NGV Characteristic
NGV is a dry burning fuel and does not contain any wear reducing additives as petrol does.
NGV enters the combustion chamber as a gas, has a far higher octane rating and generates higher combustion temperatures adding extra stress to the hot and dry valves and valve seats.

Outcomes
The lack of valve lubrication plus the higher combustion temperature of NGV often results in premature failure of exhaust valves due to a phenomenon known as Valve Seat Recession.
Then the cylinder head will have to be repaired or replaced.
Even hardened valves and valve seats are prone to fail due to corrosion and erosion when driving on NGV. Apart from gradual loss in power, increased fuel consumption and exhaust emission, the repair of a cylinder head is usually a major expenditure, this increases when a broken valve damages a piston or cylinder block.

Hidden Cost
Hidden cost If a gas converted engine runs without a valve and valve seat lubricant
In a project surveying gas engine users and cylinder head reconditioners it was revealed that the time for valve and valve seat maintenance averaged 25% of the total maintenance and repair time was as high as 75% in some systems.
Research in USA has shown that even the best valve seats (Stellite) do not prevent valve seat recession when an engine runs on dry gas without a special high temperature valve and valve seat lubricant.